1. Field of the Invention
This invention relates to a radiation shield and method for shielding an object, such as a convection section of a furnace, from radiant energy emitted by a radiant energy source, such as a radiant section of a furnace.
2. Description of Prior Art
One important modern industrial process relates to the rapid heating of essentially saturated hydrocarbons, such as ethane, propane, naphtha or gas oil to produce less saturated products, such as ethylene, propylene, butadiene, acetylene, etc. One method that is used to heat these saturated hydrocarbons is to burn a fuel; use the hot flue gases given off by the combustion of the fuel to preheat the saturated hydrocarbons; and then heat the hydrocarbons through the cracking range in close proximity to the burning fuel.
This method, commonly referred to as "steam cracking", has typically been effected by supplying the feedstock in vaporized or unvaporized form, in admixture with substantial amounts of steam, to suitable rows of tubes, known as "coils", in a cracking furnace. It is conventional to pass this reaction mixture through a number of parallel coils which pass through a convection section of the cracking furnace wherein the hot flue gas given off by the combustion of the fuel raises the temperature of the reaction mixture to some point below cracking temperature. The reaction mixture then passes through coils in a radiant section of the cracking furnace wherein burners supply the heat necessary, substantially in the form of radiant energy, to bring the reactants to the desired reaction cracking temperature and effect the desired reaction.
One problem that has imposed constraints on modern designs of steam cracking furnaces is that the convection section will "drain" or "steal" radiant energy from the radiant section to the extent that the radiant section is in the direct "line-of-sight" of the convection section. To compensate for this lost energy, additional fuel must be burned to maintain the desired temperatures in the radiant section. Of course, the greater the "field of view" between the radiant and convection sections, the greater the extent of this radiation absorption by the convection section.
Various designs have been proposed to reduce this undesirable effect, such as that disclosed in the patent to Wallace, U.S. Pat. No. 3,671,198. In Wallace the convection section is offset to the side of the radiant section to reduce or eliminate the extent to which the convection section is in direct "line-of-sight" of the radiant section so that a reduced amount of the radiant heat reaches the convection section.
Another proposed solution is to raise and separate the convection section sufficiently above the radiant section so that a long flue gas passage that connects the two sections can be used to significantly narrow the "field of view" between the two sections and thus physically shield the convection section from radiant heat given off by the radiant section.
These solutions, however, increase the cost and size of the furnace by requiring the convection section to be physically separated from the radiant section.
Thus, there is a need for a furnace which substantially reduces or eliminates the loss of radiant heat from the radiant section to the convection section, but is nevertheless simple in design.